This invention relates to a detector for providing an indication of the rate of movement of a movable member and the disclosure will be facilitated by particular reference to detecting rotational speed of a rotatable shaft. In many types of machines, apparatus and systems, it is necessary or desirable to monitor the rotational speed of a shaft or the like and to provide for an alarm that produces a signal when the rotational speed departs from a preselected range. For example, in some machines it is desirable to provide an alarm or other suitable indication to the operator when the speed of the shaft drops below a preselected amount. In agricultural equipment such as combines, planters and the like, the detection and signaling to the operator of a slow down of the speed of a shaft is important so that potential trouble can be detected and remedied before more serious difficulties or breakdown occur.